Atomic beam tube having a homogenious polarizing magnetic field in the rf transition region

ABSTRACT

An atomic beam tube has a magnetic C-field region which produces a very uniform static magnetic polarizing field transverse to the path of the atomic beam. The C-field is produced by two coils wound inside a tubular member and longitudinally aligned with the atomic beam. The two coils are spaced apart a predetermined distance. A magnetic field-producing baffle shield is disposed in the C-field region adjacent to the atomic beam for assisting in the maintenance of a uniform C-field.

United States Patent [:51 3,670,171 Lacey et al. 1 June 13, 1972 ATOMIC BEAM TUBE HAVING A [56) References Cited HOMOGENIOUS POLARIZING MAGNETIC FIELD IN THE RF 3 323 009 S Z' I PATENTS 250m 3 o oway 3,345,581 10/1967 Vessot ,.331/94 [72] Inventors: Richard F. Lacey, Peabody, Mass;

Leonard S. Cutter, Los Altos Hills; Wilson Primary Examiner-James W. Lawrence S. Turner, Los Gatos, both of Calif. Assistant Examiner-A. L. Birch Anor e Ste hen P. Fox [73] Assignee: Hewlett-Packard Company, Palo Alto, n y p Calif. 5 7] ABSTRACT [22] Ffled: June 1969 An atomic beam tube has a magnetic C-field region which [2] A L N 837,39 produces a very unifom-i static magnetic polarizing field transverse to the path of the atomic beam. The C-field is produced by two coils wound inside a tubular member and longitul Cl 1/ 1/ 4 dinally aligned with the atomic beam. The two coils are spaced [5 i) Int. Cl. ..G0ln 27/78, HOls 1/00 apart a predetermined distance. A magnetic field-producing [58] Field of Search 250/413; 331/3, 94 baffle shield is disposed in the C-field region adjacent to the atomic beam for assisting in the maintenance of a uniform C- field.

8 Claims, 4 Drawing figures FREQUENCY CONTROLLED A CONTROL MULHPUER OSCILLATOR B1RCU|TS P'A'TENTEDJun 1 3 m2 \Lv mm F14 EOFE 2M4 5.2 358 :mTEEmS: 5 305.200 Y 5255mm.

INVENTORS RICHARD F. LACEY LEONARD S. CUTLER WILSON R. TURNER BY z A AGENT ATOMIC BEAM TUBE HAVING A HOMOGEN'IOUS POLARIZING MAGNETIC FIELD IN THE RF TRANSITION REGION BACKGROUND OF THE INVENTION The present invention relates in general to atomic beam tubes of the type disclosed, for example, in US. Pat. 3,323,008, issued to Joseph H. Holloway, et al., on May 30, I967, and in co-pending US. Pat. application Ser. No. 743,839, filed in the names of Leonard S. Cutler, et al., on July l0, I968.

A typical beam tube comprises a source of molecular or atomic particles, and a first deflecting or state selecting magnet, commonly referred to as the A magnet, which selects from the source only those particles having certain energy states. The atomic particles are formed into at least one beam and transmitted into a radio frequency (r.f.) transition section of the tube, wherein the atomic particles undergo magnetic hyperfine resonance transitions, i.e., transitions from one energy state to another. This is accomplished by applying r.f. energy to the atomic particles at the transition frequency of the particles in the presence of a polarizing magnetic field. The polarizing magnetic field, commonly referred to as the C- field, should have a predetermined orientation relative to the r.f. field, and a low value relative to the magnitude of the magnetic field produced by the A magnet. The atomic particles pass from the r.f. transition section of the tube to a second deflecting or state selecting magnet which is known as the B magnet, and is similar in configuration and operation to the A magnet. The B magnet functions to direct onto a suitable detector those atomic particles which have undergone r.f. transistors. The atomic particles which do not undergo resonance transitions are directed away from the detector. Thus, by monitoring the output of the detector, it can be determined whether or not magnetic resonance has occurred.

The polarizing magnetic C-field produced in the r.f. transition section preferably should be highly uniform along the path traversed by the atomic beam. However, in many prior art devices, the C-field magnet is configured so that undesirable magnetic field inhomogeneities are produced in the beam path region. Consequently, the C- field is not well-defined, and the uncertainty in its value causes a loss in the accuracy of the beam tube when used as a frequency or time standard, for example, because the resonance frequency depends on the magnitude of the C-field. In one type of beam tube, the C-field magnet configuration includes a solenoid coil and the beam passes in close proximity thereto. With this arrangement, the inhomogeneities in the polarizing field are often due to the magnetic fringe field effects at the edges of the solenoid coil, which in turn cause a field gradient to be produced across the beam. The adverse effects of the field gradient are particularly evident in such a beam tube when a plurality of atomic beams are directed in parallel paths through the r.f. transition section, or when the cross-section of the beam is wide.

SUMMARY OF THE INVENTION The present invention relates to an atomic beam tube apparatus wherein a magnetic C-field region is configured to produce a highly uniform polarizing magnetic field along the beam path in the r.f. transition section. The preferred embodiment of the C-field magnet structure includes an elongated tubular member through which the atomic beam passes. The tubular member is made of a magnetically permeable material and comprises side walls for shielding the beam in the r.f. transition section. Disposed on at least one of the side walls of the tubular member are first and second conductive elements for conducting current longitudinally of the beam path to produce a magnetic polarizing field transverse to the beam. The first and second conductive elements each may be a solenoide coil wound around the internal side and end walls of the tubular member, for example. The two solenoid coils are spaced apart a predetermined distance to define a gap therebetween to counteract fringe magnetic field effects produced when current flows through the conductive elements. As a result, the magnetic polarizing field established along the beam path is homogeneous over a cross-section of the beam path even in the regions which are in close proximity to the conductive elements.

The r.f. transition section of the atomic beam tube includes a waveguide structure internal to the elongated tubular member for applying microwave energy to the atomic beam. The waveguide structure has two sections which are spaced apart along the beam path and an input waveguide coupled from these two sections through the tubular member to receive microwave energy from an external source. A feature of the present invention is the provision of baffling means disposed adjacent to and longitudinally of the atomic beam between the two sections of the waveguide structure for shielding the atomic beam from the magnetic field perturba tions resulting from the location of the input waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a diagrammatic illustration of an atomic beam tube including a cut-away view along the longitudinal midline of the r.f. transition section of the beam tube.

FIG. 2 is an enlarged cross-sectional view taken along the line 2-2 of FIG. 1.

FIG. 3 is a sectional view taken along the line 3-3 of FIG. 1.

FIG. 4 is a cross-sectional view similar to that of FIG. 2 and illustrating a typical prior art configuration for the C-field magnet.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, an atomic beam tube includes an oven source ll containing liquid cesium, for example. The cesium is evaporated and diffuses out of the source 11 in the form of a beam, which is directed through a first state selecting magnet structure (the A-field magnet) 13. Cesium atoms exist naturally in two different energy states, and the state selecting magnet structure 13 operates to deflect atoms of one energy state out of the beam path while directing atoms of the other desired energy state along the beam path. Atoms of the desired energy state are then directed through an r.f. transition section I4 including two waveguide sections 15, 17 which are fed from another waveguide 19. The waveguide 19 is driven at its mid-point from a source of microwave energy, as hereinafter described. If the microwave energy is at the proper frequency, the atoms subjected thereto along the beam path are caused to change energy states. The atoms then pass through a second state selecting magnet structure 2] which is configured substantially the same as the first state selecting magnet 13 and operates to deflect out of the beam path all atoms except those which have undergone a transition in energy state. The atoms having the desired energy state impinge on a detector 23. This detector produces an output signal which is fed to control circuitry 25 which in turn produces an error signal for controlling a crystal oscillator 27. The output of the oscillator 27 is multiplied by a frequency multiplier 29, which provides microwave energy to the waveguide 19 at the resonance frequency of the atoms travelling through the r.f. section.

As indicated above, if the frequency of the microwave energy equals the resonance frequency of the atoms, the atoms change energy states and are subsequently directed to the detector 23. Therefore the presence of an atom current output from the detector 23 indicates that the signal frequency injected into the waveguide 19 by the frequency multiplier 29 is equal to the resonance frequency of the atoms. The circuits 25, 27, 29 operate in a servo loop with the beam tube to maintain the microwave energy at the resonance frequency of the atoms. A precisely controlled output signal obtained from the controlled crystal oscillator 27 at an output terminal 31 may be applied to suitable external utilization circuitry.

In order for the atoms to change energy states, they must be properly polarized in the r.f. transition section 14. Referring now to FIGS. 1, 2 and 3, polarization is achieved by magnetic C-field producing means which includes an elongated tubular structure 32 surrounding the atomic beam and made of a magnetic permeable material. The tubular structure 32 has a rectangular cross-section and is formed of two opposite side walls 33, 35 and two other opposite side walls 37, 39. The ends of the tubular structure 32 are closed by end plates 41, 43 which are also made of magnetic permeable material. The end plates 41, 43 are apertured to permit the atomic beam to pass therethrough. Thus, the waveguide structures l5, l7, 19 are completely enclosed in a box formed by the tubular structure 32 and the end plates 41,43.

Disposed on the side wall 33 are first and second means for conducting current longitudinally of the beam axis to produce a polarizing magnetic C-field transversely to the direction of the beam. in the illustrated embodiment of the invention, the first and second conducting means are respectively the groups of insulated wires 45, 47 which are disposed parallel to one another and to this axis of the atomic beam on the interior surface of the side wall 33. As shown in FIG. 2, current is conducted in both groups of wires 45, 47 in a direction perpendicular to and out of the plane of the paper, as indicated by the dots in each wire according to common convention. This current flow produces a magnetic C-field, as indicated by the field lines H,, for suitably polarizing the atoms of the beam as they pass through the r.f. transition section. The atomic beam travels perpendicularly to the plane of the paper in the region indicated generally by the dashed circular outline 49.

An important feature of the present invention is the spacing of the first and second groups of wires 45, 47 relative to one another and to the side walls 37, 39. It has been found that in manufacturing the beam tube apparatus, that an unavoidable gap often occurs between the side walls and the wires closest thereto. FIG. 4 illustrates this problem in a typical prior art configuration in which the C-field H, is produced internally of a U-shaped channel member 49 by a solenoid coil 51. The gaps between the sides of the channel member and the wires of the solenoid coil closest thereto are shown at 53. These gaps cause fringe field effects in the magnetic field so that the magnetic field lines H, are not linear. The perturbations in the magnetic filed H, may appear in the region of the atomic beam and adversely affect the polarization of the atoms, with consequential broadening of the resonance line width.

In the beam tube apparatus of the present invention, the magnetic field perturbations or gradients are minimized by positioning the two groups of wires 45, 47 comprising the first and second conducting means in a predetermined spacedapart relation. With reference to FIGS. 2 and 3, it has been found the homogeneity of the magnetic field H in the region of the atomic beam is maximized if the following equation is satisfied:

where d is the distance between each of the side walls 37, 39 and the closest lateral edge of the closest one of the groups of wires 45, 47; d is the spacing between the two groups of wires 45, 47; y is the distance from the mid-plane of the wires 45, 47 to the center of the atomic beam region; and a is the width of the box between the side walls 37 39. If y/a 1, the above equation is approximately satisfied when d, 2d,. Thus, when the distances are equal, the spacing a between the two groups of wires 45, 47 should be approximately equal to the sum of the two distances d, in order to produce a highly uniform, homogeneous magnetic C-field in the atomic beam region. in the case where the atomic beam passes very close to the side wall 33, the distances d may be selected first and the distance d then computed from the above equation to arrive at a dimensional relationship which produces a homogeneous C-field.

1n the illustrated embodiment, the two groups of wires 45, 47 are each part of a solenoid coil which is wound in a rectangular loop in contiguous relationship with the interior surfaces of the side walls 33, 35 and the end plates 41, 43. Each solenoid coil is coupled to a source of power, not shown, so that current in the lower windings 45, 47' of the coils (as viewed in FIG. 2) flows in a direction opposite to current in the upper windings 45, 47. Current in the lower windings flows into the plane of the paper, as indicated by the convention of showing x's in each wire of the coils.

in order to minimize the magnetic field effects of the hole in the wall 35 of the tubular structure 32 where r.f. energy is introduced into waveguide 19 from frequency multiplier 29, a baffle plate 55 of magnetic permeable material may be positioned between the side walls 37, 39 and parallel to the side wall 35 in the region between the waveguide sections l5, 17. This baffle plate shields the atomic beam from the hole in wall 35. The baffle plate 55 has a solenoid coil 57 evenly wound thereon, as shown. Alternatively, the solenoid coil may be configured similarly to the groups of wires 45, 47. The solenoid coil 57 is coupled to a source of power not shown, so that current conducted by its upper windings is in a direction opposite to that conducted by the portions 45, 47 of the other two solenoid coils.

it is to be noted that the homogeneous magnetic C-field may be produced by means other than solenoid coils shown in drawings. For example, the wires 45, 47 may be substituted with other current conducting elements such as metallic strips positioned or deposited on the side wall 33 and electrically insulated therefrom. As in the case of the solenoid wires, the conductive strips should be configured so that the lateral edges thereof are spaced apart from one another and from the side walls 37, 39 so that the equation presented hereinabove is satisfied.

We claim:

1. In a molecular beam tube apparatus including source means for projecting molecular particles, at first state selector for forming said projected particles into at least one molecular beam, an r.f. transition section disposed downstream from said first state selector for effecting resonance of the beam particles, means for producing a polarizing magnet field in the r.f. transition section, a second state selector disposed downstream from said r.f. transition section for deflecting selected molecular particles in the beam, and a detector for receiving molecular particles from said second state selector to indicate when molecular resonance occurs in said r.f. transition section; the improvement wherein said means for producing a polarizing field comprises:

an elongated structure constructed of magnetically permeable material and having a longitudinal axis aligned with the path of said molecular beam, said structure including a closed tubular member having a rectangular cross section and being formed of a plurality of perpendicularly contiguous side and end walls configured to surround said beam in said r.f. transition section, said end walls being apertured for passage of the molecular beam therethrough; and

means for conducting current longitudinally of said elongated structure to produce a homogeneous magnetic polarizing field transverse to the path of said molecular beam, said current conducting means having first and second parallel conductive portions disposed on at least one ofsaid side and end walls in symmetrical relation with respect to a central longitudinal axis of said elongated structure, said first and second conductive portions being laterally spaced apart a predetermined distance to define a gap therebetween for counteracting fringe field effects produced by said current conducting means.

2. The apparatus of claim I,

said first and second conductive portions each being spaced apart from the adjacent wall perpendicularly contiguous to the wall on which said conductive portions are disposed by a distance d, and said conductive portions also being spaced apart from each other by a distance d such that the distance d, and d approximately satisfy the equation:

where a is the distance between opposite walls and y is the distance from the molecular beam to the plane containing said conductive portions.

3. The apparatus of claim 1,

said r.f. transition section further including a waveguide structure internal to said elongated structure and having two sections spaced apart along the molecular beam for applying microwave energy to the molecular beam;

said elongated structure having an aperture in one side wall thereof for coupling microwave energy from an external source to said waveguide structure; and

baffling means disposed adjacent to and longitudinally of the molecular beam and intermediate the two sections of said waveguide structure for shielding the molecular beam from the magnetic field effects of the aperture in said last named side wall.

4. The apparatus of claim 3, wherein said baffling means includes a solenoid coil wound thereon for producing a magnetic field in the region of the atomic beam in the same direction as the magnetic field produced by said first and second parallel conductive portions.

5. The apparatus of claim 1, wherein said first and second conductive portions are disposed on opposite ones of said side and end walls and are also spaced apart a distance substantially equal to the sum of the distances from the lateral edge of each conductive portion to the perpendicular wall closest to said lateral edge,

6, The apparatus of claim 5, wherein said first and second parallel conductive portions respectively include first and second solenoid coils disposed in contiguous relation with opposite ones of said side walls and said end walls, said solenoid coils disposed on one of said opposite side walls being connected to conduct current in a direction opposite to current in the solenoid coils disposed on the other one of said opposite side walls.

7. in a molecular beam tube apparatus including source means for projecting molecular particles, a first state selector for forming said projected particles into at least one molecular beam, an r.f. transition section disposed downstream from said first state selector for effecting resonance of the beam particles, said r.f. transition including a waveguide structure having two sections spaced apart along the molecular beam path for applying microwave energy to the molecular beam, means for producing a polarizing magnet field in the r.f. transition section, a second state selector disposed downstream from said r.f. transition section for deflecting selected molecular particles in the beam, and a detector for receiving molecular particles from said second state selector to indicate when molecular resonance occurs in said r.f. transition section; the improvement wherein said means for producing a polarizing field comprises:

an elongated structure constructed of magnetically permeable material and having a longitudinal axis aligned with the path of said molecular beam, said elongated structure being configured to surround said waveguide structure and the molecular beam in said r.f. transition section, said elongated structure having an aperture in one side wall thereof for coupling microwave energy from an external source to said waveguide structure;

means for conducting current longitudinally of said elongated structure to produce a homogeneous magnetic polarizing field transverse to the path of said molecular beam; and

baffling means disposed adjacent to and longitudinally of the molecular beam and intermediate the two sections of said waveguide structure for shielding the molecular beam from the magnetic field effects of the aperture in said side wall. 8. The apparatus of claim 7, wherein said baffling means includes a solenoid coil wound thereon for producing a magnetic field in the region of the molecular beam in the same direction as the magnetic field produced by said means for conducting current longitudinally of said elongated structure.

I t l i i UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION should read (SEAL) Attest:

EDWARD M.FLETCHER Attesting Officer Inventors:

Patent No. 3 670 7 Dated June 13 1972 Inventor(s) gjghard E. LQQQLQLA It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1 of the title page below the title of the patent Inventors:

Richard F. Lacey, Peabody, Mass. Leonard S. Cutter Los Altos Hills; Wilson S. Turner, Los Gatos, both of Calif."

Richard F. Lacey, Peabody, Mass.; Leonard S. Cutler Los Altos Hills;

Wilson R. Turner, Los Gatos both of Calif Signed and sealed this 2nd day of January 1973.

ROBERT GOTTSCHALK Commissioner of Patents FORM PC4050 110-69) U-IGOOIO USCOMM-DC 60376-F'69 w u s covtnnnlzm' PRINTING BFFICE 19" O--J6i-334 

1. In a molecular beam tube apparatus including source means for projecting molecular particles, a first state selector for forming said projected particles into at least one molecular beam, an r.f. transition section disposed downstream from said first state selector for effecting resonance of the beam particles, means for producing a polarizing magnet field in the r.f. transition section, a second state selector disposed downstream from said r.f. transition section for deflecting selected molecular particles in the beam, and a detector for receiving molecular particles from said second state selector to indicate when molecular resonance occurs in said r.f. transiTion section; the improvement wherein said means for producing a polarizing field comprises: an elongated structure constructed of magnetically permeable material and having a longitudinal axis aligned with the path of said molecular beam, said structure including a closed tubular member having a rectangular cross section and being formed of a plurality of perpendicularly contiguous side and end walls configured to surround said beam in said r.f. transition section, said end walls being apertured for passage of the molecular beam therethrough; and means for conducting current longitudinally of said elongated structure to produce a homogeneous magnetic polarizing field transverse to the path of said molecular beam, said current conducting means having first and second parallel conductive portions disposed on at least one of said side and end walls in symmetrical relation with respect to a central longitudinal axis of said elongated structure, said first and second conductive portions being laterally spaced apart a predetermined distance to define a gap therebetween for counteracting fringe field effects produced by said current conducting means.
 2. The apparatus of claim 1, said first and second conductive portions each being spaced apart from the adjacent wall perpendicularly contiguous to the wall on which said conductive portions are disposed by a distance d1 and said conductive portions also being spaced apart from each other by a distance d2, such that the distance d1 and d2 approximately satisfy the equation: where a is the distance between opposite walls and y is the distance from the molecular beam to the plane containing said conductive portions.
 3. The apparatus of claim 1, said r.f. transition section further including a waveguide structure internal to said elongated structure and having two sections spaced apart along the molecular beam for applying microwave energy to the molecular beam; said elongated structure having an aperture in one side wall thereof for coupling microwave energy from an external source to said waveguide structure; and baffling means disposed adjacent to and longitudinally of the molecular beam and intermediate the two sections of said waveguide structure for shielding the molecular beam from the magnetic field effects of the aperture in said last named side wall.
 4. The apparatus of claim 3, wherein said baffling means includes a solenoid coil wound thereon for producing a magnetic field in the region of the atomic beam in the same direction as the magnetic field produced by said first and second parallel conductive portions.
 5. The apparatus of claim 1, wherein said first and second conductive portions are disposed on opposite ones of said side and end walls and are also spaced apart a distance substantially equal to the sum of the distances from the lateral edge of each conductive portion to the perpendicular wall closest to said lateral edge.
 6. The apparatus of claim 5, wherein said first and second parallel conductive portions respectively include first and second solenoid coils disposed in contiguous relation with opposite ones of said side walls and said end walls, said solenoid coils disposed on one of said opposite side walls being connected to conduct current in a direction opposite to current in the solenoid coils disposed on the other one of said opposite side walls.
 7. In a molecular beam tube apparatus including source means for projecting molecular particles, a first state selector for forming said projected particles into at least one molecular beam, an r.f. transition section disposed downstream from said first state selector for effecting resonance of the beam particles, said r.f. transition including a waveguide structure having two sections spaced apart along the molecular beam path for applying microwave energy to the molecular beam, means for producing a polarizing magnet field in the r.f. transition Section, a second state selector disposed downstream from said r.f. transition section for deflecting selected molecular particles in the beam, and a detector for receiving molecular particles from said second state selector to indicate when molecular resonance occurs in said r.f. transition section; the improvement wherein said means for producing a polarizing field comprises: an elongated structure constructed of magnetically permeable material and having a longitudinal axis aligned with the path of said molecular beam, said elongated structure being configured to surround said waveguide structure and the molecular beam in said r.f. transition section, said elongated structure having an aperture in one side wall thereof for coupling microwave energy from an external source to said waveguide structure; means for conducting current longitudinally of said elongated structure to produce a homogeneous magnetic polarizing field transverse to the path of said molecular beam; and baffling means disposed adjacent to and longitudinally of the molecular beam and intermediate the two sections of said waveguide structure for shielding the molecular beam from the magnetic field effects of the aperture in said side wall.
 8. The apparatus of claim 7, wherein said baffling means includes a solenoid coil wound thereon for producing a magnetic field in the region of the molecular beam in the same direction as the magnetic field produced by said means for conducting current longitudinally of said elongated structure. 